1. Field of Invention
The present invention relates to a semiconductor deposition process. More particularly, the present invention relates to a tungsten deposition process.
2. Description of Related Art
In semiconductor fabrication, metal is often used to lower electrical resistance of semiconductor devices or conductive lines. One of the most commonly used metals is tungsten. Some tungsten exists as tungsten silicide material inside a semiconductor product while other tungsten exists as pure tungsten material inside semiconductor structure such as a tungsten plug. In general, tungsten is deposited in a low-pressure chemical vapor deposition (LPCVD). The deposition process can be further sub-divided into three different stages including a crystal growth stage, an intermediate stage and a main deposition stage. In the growth stage, tungsten hexafluoride (WF6) and silane (SiH4) are used as reactive gases. The rate of growth of the tungsten layer is rather low. In the main deposition stage, tungsten hexafluoride (WF6) and hydrogen (H2) are used as the reactive gases to deposit tungsten at a higher deposition rate over the slow-growth crystalline layer. Hence, a thicker layer of tungsten is formed. Conventionally, tungsten deposition process also includes an intermediate stage for supplying or terminating the supply of some gases and adjusting gas pressures. In this stage, the supply of reactive gases such as tungsten hexafluoride (WF6) and silane (SiH4) are cut off.
To improve the quality of tungsten deposition, a supply of nitrogen has been proposed in U.S. Pat. No. 5,028,565 so that reflectivity of the deposited tungsten layer is higher. In other words, a smoother tungsten surface is obtained. However, since no tungsten hexafluoride (WF6) and silane (SiH4) are passed during the intermediate stage, the nitrogen molecules (N2) passed into the reaction chamber may occupy the attachment points on the crystalline growth surface of the tungsten layer. In the subsequent main deposition, tungsten deposition reaction will occur only after the nitrogen molecules on the crystalline growth surface has been replaced by reactive gases. Consequently, tungsten deposition rate will be lowered because of this delay reaction or incubation period. In addition, the nitrogen molecules are most likely to leave the tungsten growth surface at different times. Therefore, the resultant tungsten layer may have non-uniform thickness.
Furthermore, by shutting off the supply of nitrogen in the intermediate stage, non-uniformity of the deposited tungsten layer in a silicon wafer and between different silicon wafers will also improve as proposed in U.S. Pat. No. 6,036,366.
Accordingly, one object of the present invention is to provide a tungsten deposition process having a higher depositing rate and capable of producing a tungsten layer with a uniform thickness. First, a crystal growth step is conducted inside a reaction chamber to form a tungsten crystal layer using tungsten hexafluoride, silane and nitrogen as reactive gases. An intermediate step is conducted by cutting off the supply of tungsten hexafluoride to the reaction chamber but continuing the supply of silane. Meanwhile, nitrogen is selectively supplied. Finally, a main deposition step is conducted inside the reaction chamber to form a tungsten layer over the tungsten crystal layer using tungsten hexafluoride, silane and nitrogen as reactive gases.
This invention also provides an alternative tungsten deposition process having a higher depositing rate and capable of producing a tungsten layer with a uniform thickness. First, a crystal growth step is conducted inside a reaction chamber to form a tungsten crystal layer using tungsten hexafluoride, silane and nitrogen as reactive gases. The supply of tungsten hexafluoride to the reaction chamber is cut off first while the supply of silane is cut off after a defined period, thereby ending the crystal growth step. Within the defined period, gaseous nitrogen is passed into the reaction chamber selectively. In addition, the defined period must be long enough to permit silane molecules to occupy all the attachment points on the crystal growth surface of the tungsten layer. An intermediate step is conducted without passing any tungsten hexafluoride and silane into the reaction chamber. Finally, a main deposition step is conducted inside the reaction chamber to form a tungsten layer over the tungsten crystal layer using tungsten hexafluoride, silane and nitrogen as reactive gases.
According to experiments, the silane passed into the reaction chamber in the intermediate step is capable of occupying all the attachment points on the tungsten crystal layer even in the presence of nitrogen. Reaction between tungsten hexafluoride and silane molecules on the tungsten crystal layer can start immediately in the main deposition step because all nitrogen molecules have already been displaced by silane. By eliminating the incubation period necessary for removing nitrogen, a higher deposition rate of tungsten and a uniform tungsten layer is obtained.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.